Thin film transistors (hereinafter also referred to as TFTs) formed over a flat plate such as a glass substrate, which are typically used in liquid crystal display devices, are generally formed using semiconductor materials such as amorphous silicon or polycrystalline silicon. Although TFTs formed using amorphous silicon have low field-effect mobility, they have an advantage that larger glass substrates can be used. Meanwhile, TFTs formed using polycrystalline silicon have high field-effect mobility; however, they need to be subjected to a crystallization step such as laser annealing and thus are not always suitable for larger glass substrates.
On the other hand, TFTs formed using oxide semiconductors as semiconductor materials have attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used for a switching element in an image display device.
A TFT in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have a higher electric field mobility than a TFT formed using amorphous silicon. An oxide semiconductor film can be formed with a sputtering method or the like and thus, fabrication of the TFT using an oxide semiconductor is easier than that of the TFT using polycrystalline silicon.
TFTs formed using such an oxide semiconductor are expected to be applied to switching elements included in pixel portions and driver circuits of display devices such as a liquid crystal display, an electroluminescent display (hereinafter also referred to as an EL display), and electronic paper. For example, Patent Document 3 discloses a technique in which a pixel portion and a driver circuit of a display device are formed using TFTs each formed using the oxide semiconductor described above.